1. Field of the Invention
The present invention relates to techniques for the treatment of image signals.
Particular attention has been paid to the possibility of applying embodiments to compression of RGB images on 24 bits per pixel with a view to their subsequent storage in a frame buffer. The reference to this particular type of application must not, however, be understood as in any way limiting the scope of the invention, which is altogether general.
2. Description of the Related Art
In different sectors of the art there arises the need to reduce the number of bits for representing a color since some displays have a limited number of bits per color, in particular color images in graphic-display cards for personal computers and workstations, or in printers with a limited availability of colors.
For example, in the case of images in RGB format, a true-color image is made up of 8 bits for each chromatic component, for a total of 24 bits per pixel.
Known in the literature are solutions, based upon the “dithering” principle, which enable simulation of colors that are not available in the display color pallets. This result is obtained by arranging the adjacent pixels with different colors from those of the image source in a configuration such as to simulate the colors of the image source itself that are not available. This technique exploits the tendency of the human eye to mix colors when it is observing complex configurations of colors.
A typical effect of dithering may be encountered on television displays or in typographical prints. At a certain distance, the images seem to comprise many colors or many different shades or nuances of color, but when they are observed in greater detail, it is found not to be the case. For example, a television image, even though it may appear to have a continuous tone, uses only three colors set in various states of activation/deactivation. Typographical print, for example newspaper print, uses only black ink; however, the images appear to be made up of grey tones.
Fundamentally, the principle of dithering is based upon the simulation of an intermediate tone by mixing, in appropriate amounts (for example as a result of different geometrical distributions), two colors, for instance, black and white for simulation of an entire range of grey.
The prospect of developing dithering techniques that may enable a high ratio of compression with satisfactory results at the display level is today particularly felt.
For example, in 3D graphic-display cards for mobile telephones there is felt the need to provide an acceptable display, starting from image data organized in 4 or 5-6 bits per pixel (for color images), or even starting from only one bit per pixel for monochromatic displays.